Based on recent incidents with the unintentional detonation of munitions, there has been increased research into the area of creating better, more efficient IM. Most of these newer systems rely on passively activated designs to vent their explosive material to the environment before environmental conditions can activate the propellants or munitions.
Some systems that have been developed use explosive products to create holes in the munitions or propellant containers to vent them. The explosive products or the impact of a bullet, fragment, or shaped charge jet from the systems may result in a temperature sufficient to ignite the propellants or munitions. Additionally the explosive products or the impact of a bullet, fragment, or shaped charge jet may inadvertently initiate the venting system thereby rendering the munitions ineffective.
In U.S. Pat. No. 6,360,526 issued to Kunstmann on Mar. 26, 2002 such an passive system is described. A rocket motor having a desensitizing mechanism for preventing explosion or violent reaction during slow cook off is disclosed. In Kunstmann the rocket motor includes a case contained rocket propellant with a desensitizing assembly attached forward of the propellant charge. The desensitizing assembly is formed with an enclosure containing a desensitizing fluid, and connected to the interior of the rocket motor by a tube which is sealed by a heat sensitive plug. The heat sensitive plug melts at a temperature below the slow cook off temperature of the rocket propellant and the melting of the plug allows the desensitizing fluid to be injected into the interior of the casing and onto the propellant charges, thereby, desensitizing the propellant charge.
In U.S. Pat. No. 6,338,242 issued to Kim et. al on Jan. 15, 2002 a similar passive system is disclosed. In Kim, an ordnance venting system has an ordnance device having a casing with a vent opening, a dome plug fitted into the formed vent opening and an adapter fitted over the dome plug on the outside of the casing. The adapter connects sufficiently to the casing to retain the dome plug against the formed vent opening for given pressures. The adapter melts at high temperatures and releases the dome plug to reduce the danger of explosion from heat induced over pressurization.
One system utilizing a charge is described in U.S. patent application Ser. No. 11/261,184 filed Oct. 28, 2005 by Skinner. Skinner discloses a device for venting a container housing and energetic material including an installation portion, a charge holder disposed in the insulating portion, and an explosive cutting charge disposed in the cardholder. The device further includes a thermally activated initiation device, and a transfer line coupling the thermally activated initiation device and the explosive cutting charge. When exposed to a temperature at or above a predetermined temperature heat produced deflagration of the thermally activated initiation device initiates deflagration in the transfer line which in turn detonates explosive cutting charges. Upon detonation these explosive cutting charges perforate the container to relieve pressure or avoid buildup of pressure within the case.
Fuses, charges or plugs do allow venting however they are not as durable and they do not allow as easily for normal operation as the presently disclosed device. They also do not offer a lot of venting; therefore many plugs, fuses or charges may be required.
For the purposes of this application the term Shape Memory Polymers as described below is equivalent to Dynamic Modulus Resins.
Shape memory materials were first developed about 20 years ago and have been the subject of commercial development in the last 10 years. Shape memory materials derive their name from their inherent ability to return to their original “memorized” shape after undergoing a shape deformation. There are principally two types of shape memory materials, shape memory alloys (SMAs) and shape memory polymers (SMPs).
SMAs and SMPs that have been preformed can be more easily deformed to a desired shape above their glass transition temperature (Tg). The SMA and SMP must remain below, or be quenched to below, the Tg while maintained in the desired shape to “lock” in the deformation. Once the deformation is locked in, the SMA, because of its crystalline network, and the SMP, because of its polymer network, cannot return to a relaxed state due to thermal barriers. The SMA and SMP will hold its deformed shape indefinitely until it is heated above its Tg, whereupon the SMA and SMP stored mechanical strain is released and the SMA and SMP returns to its preformed state.
There are principally two types of plastics, thermoset resins and thermoplastic resins, each with its own set of unique characteristics. Thermoset resins, for example polyesters, are liquids that react with a catalyst to form a solid, and cannot be returned to their liquid state, and therefore, cannot be reshaped without destroying the polymer networks. Thermoplastics resins, for example PVC, are also liquids that become solids. But unlike thermoset resins, thermoplastics are softened by application of heat or other catalysts. Thermoplastics can be heated, reshaped, heated, and reshaped over and over.
SMPs used in the presently disclosed device are unique thermosetting polymers that, unlike traditional thermosetting polymers, can be reshaped and formed to a great extent because of their shape memory nature and will not return to a liquid upon application of heat. Thus by creating a shape memory polymer that is also a thermosetting polymer, designers can utilize the beneficial properties of both thermosetting and thermoplastic resins while eliminating or reducing the unwanted properties. Such polymers are described in U.S. Pat. No. 6,759,481 issued to Tong, on Jul. 6, 2004 which is incorporated herein by reference. Other thermoset resins are seen in PCT Application No. PCT/US2006/062179, filed by Tong, et al on Dec. 15, 2006; and PCT Application No. PCT/US2005/015685 filed by Tong et al, on May 5, 2005 of which both applications are incorporated herein by reference.
There are three types of SMP's: 1) A partially cured resin, 2) thermoplastics, and 3) fully cured thermoset systems. There are limitations and drawbacks to the first two types of SMP. Partially cured resins continue to cure during operation and change properties with every cycle. Thermoplastic SMP “creeps,” which mean it gradually “forgets” its memory shape over time. A thorough understanding of the chemical mechanisms involved will allow those of skill in the art to tailor the formulations of SMP to meet specific needs, although generally fully cured thermoset resin systems are preferred in manufacturing.
While SMA and SMP appear to operate similarly on the macro scale, at the molecular scale it is apparent that the method of operation of each is very different. The difference between SMA and SMP at the molecular level is in the linkages between molecules. SMA essentially has fixed length linkages that exist at alternating angles establishing in a zigzag patterned molecular structure. Reshaping is achieved by straightening the angled connections from alternating angles to straight forming a cubic like structure. This method of reshaping SMA material enables bending while limiting any local strains within the SMA materials to less than 8% strain, as the maximum shape memory strain for SMA is 8%. This 8% strain allows for the expansion or contraction of the SMA by only 8%, a strain that is not useful for most industrial applications. Recovery to memory shape is achieved by heating the material above a certain temperature at which point the molecules return to their original zigzag molecular configuration with significant force thereby reestablishing the memory shape. The molecular change in SMA is considered a metallic phase change from Austensite to Martensite which is defined by the two different molecular structures.
SMP has connections between molecules with some slack. When heated these links between connections are easily contorted, stretched and reoriented due to their elastic nature as the SMP behaves like an elastic material when heated, when cooled, the shape is fixed to how it was being held. In the cooled state the material behaves as a typical rigid polymer that was manufactured in that shape. Once heated the material again returns to the elastic state and can be reformed or return to the memory shape with very low force. Unlike SMA which possesses two different molecular structures, SMP is either a soft elastomer when heated or a rigid polymer when cool. Both SMA and SMP can be formulated to adjust the activation temperature for various applications. Critical to the success of the currently claimed device is thermoset SMP which provides an order of magnitude higher stiffness than previous state-of-the-art thermoplastic SMPs. This added stiffness coupled with high strain capability enables the development and use of a highly useful composite tooling technology.
Shape memory alloys have been used to attempt to solve the IM issue as disclosed in U.S. Pat. No. 6,321,656 issued to Johnson on Nov. 27, 2001. In Johnson a thermally responsive material, such as Nitinol, an SMA, is used to create a latching mechanism which, upon exposure to temperatures in excess of the Tg of the Nitinol, the latching mechanism changes shape so as to mechanically unlatch the rocket casing section holding the propellant. However, as noted above the shape change is minimal requiring very large mechanisms to ensure unlatching or thinner SMA as the latching mechanism which may fail due to a lack of structural strength. Additionally Johnson does not describe or disclose the use of an SMP or SMP composite for use in a venting system for IM.
The term “composite” is commonly used in industry to identify components produced by impregnating a fibrous material with a thermoplastic or thermosetting resin to form laminates or layers. Generally, polymers and polymer composites have the advantages of weight saving, high specific mechanical properties, and good corrosion resistance, which make them indispensable materials in all areas of manufacturing. Nevertheless, manufacturing costs are sometimes detrimental, since they can represent a considerable part of the total costs and are made even more costly by the inability to quickly and easily repair these materials without requiring a complete, and expensive, total replacement. Furthermore, the production of complex shaped parts is still a challenge for the composites industry. Because SMPs are resins, they can be used to make composites, which are referred to in this application as SMP composites.
Unlike SMAs, SMPs exhibit a radical change from a normal rigid polymer to a flexible elastic and back on command. SMA would be more difficult to use for thermally activated venting systems for rocket motors because SMAs do not have the ease in changing the activation temperature as do SMP's for different propellants. SMAs would also have issues with galvanic reactions with other metals which would lead to long term instability. The current supply chain for SMAs is currently not consistent as well. SMP materials offer the stability and availability of a plastic and are more inert than SMAs. Additionally, when made into a composite SMPs offer similar if not identical mechanical properties to that of traditional metals and SMAs in particular. Throughout this disclosure SMP and SMP composites are used interchangeably as each can be replaced by the other depending on the specific design requirements to be met.
Therefore there is a need for a low cost, high strain system to allow for the easy venting of IM so as to prevent inadvertent or slow cook off of stored munitions.